The EGF receptor (EGFR) has been the prototype for growth factor receptor studies since its identification and sequencing as the first transmembrane tyrosine kinase. Three other family members, HER2, HERS and HER4 are highly related by topology and mechanism of activation. The growth stimulatory properties of the EGFR family are well documented, and EGFR and HER2 have been successfully targeted for human cancer therapy. However, studies in lower organisms, mammalian cells and animal models suggest a role for the EGFR family in differentiation or growth cessation. Recent work, including our own, suggests that HER4 is involved in anti-proliferative and differentiation signaling in epithelial cells. Furthermore, HER4 expression in human breast cancers correlates with a good prognosis, i.e.HER4 may have tumor suppressor capabilities. This grant's objectives are to define the unique signaling capabilities that distinguish HER4 from EGFR, HER2 and HERS and to elucidate the intracellular, "tumor suppressor" pathways by which HER4 promulgates its anti-proliferative and/or differentiation effects. In doing so, we hope to find new markers for breast cancer prognosis and new avenues for therapeutic intervention.A recent finding sets HER4 apart from other EGFR family members; ligand binding initiates a two-step HER4 proteolytic process that releases a soluble, intracellular 80 kd fragement (s80HER4). This kinase active fragment can localize in the cell nucleus due to freatures unique to HER4 among the EGFR family, distinct nuclear localization and nuclear export sequences. Little is known regarding S80HER4function, the role of its tyrosine kinase or the consequences of its nuclear action. It is our hypothesis that the production and nuclear action of S80HER4are critical steps in HER4-mediated growth inhibition and differentiation. We have created cell and mouse models to test our hypotheses and elucidate mechanisms. We will do so through the following specific aims: Aim 1 - To determine if S80HER4 production and nuclear localization are central to HER4-mediated growth inhibition; Aim 2 - To determine if differentiation of mammary epithelial cells is enhanced by S80HER4 and whether that effect requires nuclear location; Aim 3 - To inducibly express S80HER4 in mouse mammary epithelium and determine whether s80HER4 expression alters mouse mammary development and protects against breast cancer formation in vivo.